The present invention relates to a tool and method for simultaneously detecting hydrocarbons and naturally occurring gamma rays.
Hydrocarbon logging systems are well known in the art and conventionally include a logging tool adapted for movement through the well bore under the control of a surface unit. The well tool customarily contains a radioactive source and a scintillation detector responsive to radiation reflected from and/or emitted from the formation as a result of radiation from the radioactive source. The quantitative hydrocarbon information is generally contained in the amplitude and rate of the pulse output signal from the scintillation detector and this pulse signal is transmitted to the surface over the logging cable. Constant amplitude reference pulses are also generated in the tool at a predetermined frequency and these reference pulses are also transmitted to the surface via the logging cable. The signal received at the surface is generally adjusted in amplitude to restore the reference pulses to a predetermined constant amplitude in an automatic gain control circuit to thereby compensate the signal for amplitude variations due to temperature changes, etc. The reference pulses are thereafter removed from the signal and the remaining signal is evaluated as to pulse amplitude and rate to provide the hydrocarbon information.
Naturally occurring gamma ray detectors are also well known and such systems generally include a logging tool adapted for movement through the well bore and containing a scintillation detector responsive to the gamma radiation naturally emitted by the formation. This desired information is generally contained in the pulse rate of the pulse signal from the scintillation detector.
Both of the above detectors are extremely useful and it is desired to achieve both the hydrocarbon data and the natural gamma ray data in a single pass through the well bore to thereby achieve a significant savings in both time and effort. In addition, the obtaining of both naturally occurring gamma ray information and the hydrocarbon information simultaneously eliminates any errors in the correlation as a function of depth of the two logs when separately run.
The mutual interference of the two signals when sent uphole over the same logging cable has in the past proscribed the simultaneous gamma ray and hydrocarbon logging of a well bore and it is accordingly an object of the present invention to obviate the deficiencies of known prior art systems and to provide a novel logging tool and method for the simultaneous detection of hydrocarbons and naturally occurring gamma rays within a well bore.
Another object of the present invention is to provide two scintillation detector well logging tool and method in which the output signals therefrom are transmitted to the surface over a single cable with a minimum of interference.
A further object of the present invention is to provide a novel method and well logging system for hydrocarbons and naturally occurring gamma rays in which the number of components and circuit complexity are minimized over that required for a single function logging tool.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from the claims and from the following detailed description when read in conjunction with the appended drawings.